Tone-control circuit-arrangement for use in low-frequency amplifier



June 26, 1956 e. R. RICHTER 2,752,432

TONE-CONTROL CIRCUIT-ARRANGEMENT FOR USE IN LOW-FREQUENCY AMPLIFIER Filed April 22, 1953 INVENTOR GUSTAV RUDOLF R ICH ER AGENT "roNa-ooNrRor. CIRCUIT-GEMENT FOR USE IN LOW-FREQUENCY AMPLIFIER Gustav Rudolf Richter, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application April 22, 1953, Serial No. 350,292 Claims priority, application Netherlands May 21, 1952 3 Claims. Cl. 179-171 The invention relates to a tone-control circuit-arrangement for use in a low-frequency amplifier, in which a signal source is included in the input circuit of the first of two cascade-connected amplifying tubes, provided each with an unbypassed cathode resistor, these cathode resistors being connected to one another by an intercoupling resistor for producing a positive feed-back and in which at least the high-tone frequency range of the signal oscillations is derived from a tapping of this intercoupling resistor through a suitable capacitor.

In such a circuit-arrangement tone-control could be obtained in a simple manner by interrupting at will the connection between the said capacitor and the said tapping by means of a switch. It is found, however, that in this case the stability of the circuit-arrangement is harmfully affected.

The invention provides a difierent solution, in which the stability of the circuit-arrangement is not reduced. It is characterized in that the said input circuit comprises a switch by which the grid of the first tube is connected through an impedance to the said capacitor and by means of which the low-tone frequency range of the amplified signal may be attenuated.

In order that the invention may be readily carried into effect, it will now be described in detail with reference to the accompanying drawing.

Referring to the figure reference numerals 1 and 2 designate two cascade-connected amplifying tubes, in which signal oscillations produced at the movable contact 3 of a volume-control device, provided with psychological tone-control, are supplied to the input circuit of the first tube of the cascade. The two tubes 1 and 2 are provided each with a non-decoupled cathode resistor 5 and 6 respectively, which are interconnected by an intercoupling resistor 7, a tapping 8 of which is connected through a capacitor 9 to a point of constant potential or, if desired, a point of positive or negative feed-back.

The negative feedback due to the cathode resistors 5 and 6 is, in this case, compensated wholly or for the major part by the positive feed-back produced by the resistor 7. The capacitor 9, which permits at least the high-tone frequency range to pass, produces, however, a reduction of the positive feedback for these high-tone frequencies, so that by suitable proportioning of the resistors 5, 6 and 7 the low-tone frequencies are materially more amplified than the high-tone frequencies.

If it is desired to provide tone-control in such a circuitarrangement, the connection of the capacitor 9 to the point of constant potential could be interrupted by means of a switch, so that the high-tone frequencies are prevented from flowing away across this capacitor and a uniform positive feed-back, is obtained throughout the frequency range via the resistor 7. However, since the risk of self-oscillation is materially greater for high frequencies than for low frequencies, the stability of the circuit-arrangement is thus greatly affected.

States Patent 21,752,432 Patented June 26, 1956 'ice ' as a negative feed-back voltage, to the grid circuit of the tube 1. If suitably proportioned, the latter voltage exceeds the positive feed-back voltage, which is supplied via the resistor 7 to the cathode of the tube 1, so that, if the switch arm of the switch 10 is connected to the contact a, the low-tone frequency range is considerably attenuated compared with the case in whihthe switch is open. 1 p

In order to prevent an excessive attenuation of the negative feedback voltage, thus supplied to thelgrid of the tube 1, owing to the natural impedance of the signal oscillation source, i. e. owing to the impedance of the volume-control device 4, provision is made of a comparatively small capacitor 12 in the series branch of the input circuit of the tube ll; this capacitor 12 is shunted by the switch arm and a second contact b. If the switch 10 is opened this capacitor 12 produces also an attenuation of the low-tone frequency range compared with the case in which the switch arm is connected to the contact b. Thus an effective control of this low-tone frequency range is obtained.

The control of the high-tone frequencies may be carried out in a simple manner by means of a potentiometer 13, which is connected in parallel with the secondary winding of an output transformer 14 of the tube 2, a tapping of this winding being connected to a point of constant potential, the movable contact of the potentiometer 13 being connected through a capacitor 15, which allows mainly the high-tone frequencies to pass,'t0 the cathode of the tube 1 or a similar suitable point in the circuit-arrangement.

In a preferred embodiment, which is given only by way of example, the capacitor 9 has a value of 47,000 pf., the capacitor 12 a value of 1000 pf., the left-hand part of the resistor '7 a value of 18K ohms, the right-hand part a value of 12K ohms, the resistor 11 a value of 5.6M ohms, the resistor 5 a value of 1.8K ohms and the resistor 6 a value of ohms.

What is claimed is:

1. A low-frequency signal amplifier circuit provided with tone control comprising a first electron discharge tube amplifier having an input circuit, an output circuit and an unbypassed cathode resistor, a second electron discharge tube amplifier having an unbypassed cathode resistor and an input circuit coupled to the output circuit of the first amplifier, a resistor element intercoupling said cathode resistors for producing a positive feedback between the second and first amplifiers, a capacitor connected between a point on said element and a point of constant potential to by-pass the high-tone range of said signal, a negative feedback impedance, and a switch connecting the input circuit of said first amplifier through said impedance to said point of said element to effect attenuation of the low-tone range of said signal.

2. A low-frequency signal amplifier circuit provided with tone control comprising a first amplifier including a first electron discharge device having an anode, a cathode, and a control grid, a first unbypassed resistor connected to said cathode, an input circuit coupled to said grid, and an output circuit coupled to said anode, a second amplifier including a second electron discharge device having a cathode and a control grid, a second unbypassed resistor connected to the cathode of said second device, and means for connecting said output circuit to the grid of said second device, a resistor element intercoupling said cathodes for producing positive feedback between the second and first amplifiers, a capacitor connected between a point on said element and a point of constant potential to by-pass the high-tone range of said signal, a negative feedback resistor, and a switch connecting the grid of said first device through said negative feedback resistor to said point of said element to effect attenuation of the low-tone range of said signal,

3. A low-frequency signal amplifier circuit provided with tone control comprising a first amplifier including a first electron discharge device having an anode, a cathode, and a control grid, 8. first unbypassed resistor connected to said cathode and a point of constant potential,

an input capacitor, an input circuit coupled through said coupling said cathodes for producing positive feedback between the second and first amplifiers, a capacitor connected between a tap on said element and said point to by-pass the high-tone range of said signal, a negative feedback impedance, and a switch selectively connecting in one instance the grid of said first device through said impedance to said tap of said element to effect attenuation of the low-tone range of said signal and in another instance a shunt across said input capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,072,708 Case Mar. 2, 1937 2,313,098 Shepard Mar. 9, 1943 2,538,488 Volkers Jan. 16, 1951 2,572,544 Wallin Oct. 23, 1951 

